Warren L. J. Fox

Underwater acoustic communication by passive-phase conjugation: theory and experimental results

A new method for coherent underwater acoustic communication called passive phase conjugation is evaluated. The method is so named because of conceptual similarities to active phase conjugation methods that have been demonstrated in the ocean. In contrast to active techniques, however, the array in passive phase conjugation needs only receive. The procedure begins with a source transmitting a single probe pulse. After waiting for the multipathed arrivals to clear, the source then transmits the data stream. At each element in the distant receiving array, the received probe is cross-correlated with the received data stream. This cross-correlation is done in parallel at each array element and the results are summed across the array to achieve the final communication signal suitable for demodulation. As the ocean changes, it becomes necessary to break up the data stream and insert new probe pulses. Results from an experiment conducted in Puget Sound near Seattle are reported. Measurements were made at multiple ranges and water depths in range-dependent environments.

Mechanisms for subcritical penetration into a sandy bottom: Experimental and modeling results

This paper presents preliminary results of a recent study whose overall objectives are to determine the mechanisms contributing significantly to subcritical acoustic penetration into ocean sediments, and to quantify the results for use in sonar performance prediction for the detection of buried objects. In situ acoustic measurements were performed on a sandy bottom whose geoacoustical and geomorphological properties were also measured. A parametric array mounted on a tower moving on a rail was used to insonify hydrophones located above and below the sediment interface. Data covering grazing angles both above and below the nominal critical angle and in the frequency range 2-15 kHz were acquired and processed. The results are compared to two models that account for scattering of sound at the rough water-sediment interface into the sediment. Although all possible mechanisms for subcritical penetration are not modeled, the levels predicted by both models are consistent with the levels observed in the experimental data. For the specific seafloor and experimental conditions examined, the analysis suggests that for frequencies below 5-7 kHz sound penetration into the sediment at subcritical insonification is dominated by the evanescent field, while scattering due to surface roughness is the dominant mechanism at higher frequencies.

Multichannel equalization by decision-directed passive phase conjugation: experimental results

An adaptive technique for underwater acoustic communication using passive phase conjugation (PPC) is developed. Multipath channel-parameter identification is accomplished by decision-directed model building and finite-window block-updated least squares computed by LSQR (an iterative linear systems solver). The resulting channel estimates are then used by the PPC processor to generate decisions for use in the next processing block. This architecture effectively accomplishes array equalization with low computation cost in shallow-water environments that exhibit rapidly fluctuating multipath scattering. The performance on shallow-water acoustic communications channels is demonstrated at ranges of 0.9-4.6 km under windy surface conditions and shipping noise, using measured wide-band telemetry data with binary phase-shift keying signaling. The algorithm is evaluated with sparse receiver apertures using subsets of a 14-element array.

System Design Considerations for Undersea Networks: Link and Multiple Access Protocols

We address several inter-related aspects of underwater network design within the context of a cross-layer approach. We first highlight the impact of key characteristics of the acoustic propagation medium on the choice of link layer parameters; in turn, the consequences of these choices on design of a suitable MAC protocol and its performance are investigated. Specifically, the paper makes contributions on the following fronts: a) Based on accepted acoustic channel models, the pointto- point (link) capacity is numerically calculated, quantifying sensitivities to factors such as the sound speed profile, power spectral density of the (colored) additive background noise and the impact of boundary (surface) conditions for the acoustic channel; b) It provides an analysis of the Micromodem-like linklayer based on FH-FSK modulation; and finally c) it undertakes performance evaluation of a simple MAC protocol based on ALOHA with Random Backoff, that is shown to be particularly suitable for small underwater networks.

Dual-frequency acoustic camera: a candidate for an obstacle avoidance, gap-filler, and identification sensor for untethered underwater vehicles

The Dual-Frequency Identification Sonar (DIDSON) is a forward-looking sonar that can mount on an untethered underwater vehicle (UUV). It performs three important tasks. In the low-frequency mode, it ensonifies the gap between the coverage of two side-scan sonars during surveys and can serve as an obstacle avoidance sonar. In the high-frequency mode, its very high resolution allows the identification of objects in turbid water where optical systems fail. The sonar is small, light, and requires only 30 watts to operate. DIDSON currently is used on three UUVs (two swimmers and one crawler) as part of the Office of Naval Research Undersea, Autonomous Operation Capabilities Program. DIDSON has a 29/spl deg/ field of view and operates at either 1.0 MHz or 1.8 MHz. The Woods Hole REMUS vehicle, in its dual side-scan sonar configuration, has a 6-m to 8-m gap in its coverage. This gap is filled by DIDSON when looking down-range at distances greater than 16 m. The Bluefin Robotics UUV operated by the Coastal Systems Station swims in deeper water, flies higher off the bottom and has a side-scan gap up to 20 m wide. A modified DIDSON that operates at 750 kHz (DIDSON-LR) is proposed for this application. It should image at ranges in excess of 40 m. When operating as a gap-filler, DIDSON collects data at a constant frame rate and stores that data during the duration of the mission. An analysis application is being written to sift through the gigabytes of stored data, locate objects on the seafloor and score them with respect to their mine-like characteristics. Operation efficiency will dramatically increase when UUVs can identify mines autonomously and act upon these identifications. Algorithms are being developed to perform this autonomous identification. The process starts with image processing to extract salient object features. The current approach compares these features to a knowledge base of object features, allowing for object rotation and interaction with the environment. Intelligent algorithms will be developed to associate the object under consideration to objects in the knowledge base in a statistically significant way.

Distributed sensor placement with sequential particle swarm optimization

Sequential particle swarm optimization (S-PSO) is a modification of PSO suitable for high-dimensional optimization problems. S-PSO iteratively optimizes the objective function over randomly selected subspaces of the parameter search space instead of the entire parameter space at once as in standard PSO. This approach is advantageous since fewer particles are needed to solve the lower-dimension subproblems. S-PSO is applied to the distributed sonar sensor placement problem, where not only the dimensionality issue arises, but also the computational complexity of the objective function increases with the problem size. Simulations show that S-PSO outperforms standard PSO both in terms of convergence and computational efficiency.

Wide area ocean networks: architecture and system design considerations

Wide area ocean networks for monitoring and scientific exploratory purposes are in various stages of design; small-scale networks are already in various stages of deployment and testing. Clearly, cost-effective coverage is a primary underlying principle; arguably, such networks must therefore employ low-cost, energy-efficient mobile nodes. The first objective of this work is to broadly describe the architecture and system design considerations of such wide-area networks with mobile nodes; secondly, we introduce the APL/UW Seaglider capabilities and provide energy estimates for propulsion and data communications. We also discuss tradeoffs, and applications in ocean coverage, and optimization of sensor coverage within constraints of a power-efficient network.

The Kauai Experiment

The Kauai Experiment was conducted from June 24 to July 9, 2003 to provide a comprehensive study of acoustic propagation in the 8–50 kHz band for diverse applications. Particular sub‐projects were incorporated in the overall experiment 1) to study the basic propagation physics of forward‐scattered high‐frequency (HF) signals including time/angle variability, 2) to relate environmental conditions to underwater acoustic modem performance including a variety of modulation schemes such as MFSK, DSSS, QAM, passive‐phase conjugation, 3) to demonstrate HF acoustic tomography using Pacific Missile Range Facility assets and show the value of assimilating tomographic data in an ocean circulation model, and 4) to examine the possibility of improving multibeam accuracy using tomographic data. To achieve these goals, extensive environmental and acoustic measurements were made yielding over 2 terabytes of data showing both the short scale (seconds) and long scale (diurnal) variations. Interestingly, the area turned out...

Stochastic resonance of a threshold detector: image visualization and explanation

Stochastic resonance is said to occur when just the right amount of noise enhances the performance of a process. For a simple threshold detector, the first moment of stochastic resonance is obtained by passing the signal through a transfer function equal to a transposed and shifted version of the underlying noises probability distribution function. The process is readily evident in images wherein noise corresponding to a linear transfer function produces a better visual representation than when other noise is used.

Environmentally Adaptive Sonar Control in a Tactical Setting

Automatic environmentally adaptive sonar control in littoral regions characterized by high spatial/temporal acoustic variability is an important operational need. An acoustic model-based sonar conroller requires an accurate model of how the sonar would perform in the current environment while in any of its possible configurations. Since high-fidelity acoustic models are computationally intensive, and finding the optimal sonar mode may require a large number of these model runs, such a controller may not be able to provide optimal line-up solutions in tactically useful time frames. We have explored a method of statistically characterizing a given operations area, generating a large ensemble of acoustic model runs, and training specialized artificial neural networks to emulate acoustic model input/output relationships. The neural networks reproduce the acoustic model outputs to a good degree of accuracy in a small fraction of the compute time needed for one of the original model runs. In this paper, the neural network training method is described, examples of neural network performance are given, and an example of controller solutions in a variable environment are presented.